1. Field of the Invention
Ethynylated aromatic compounds, more specifically, ethynylated biphenyl compounds, are monomers which are intermediates in the synthesis of polymers having intramolecular cyclization (IMC) capability. Such polymers include, for example, imides, quinoxalines, benzimidazoles, amides and imines. The compound 2,2'-bis(phenylethynyl)-5,5'-diaminobiphenyl is also a key intermediate in the synthesis of the 4,4'5,5'-tetramino analog which is needed to make quinoxaline and benzimidazole polymers.
Nitronium trifluoromethanesulfonate is especially effective in selectively nitrating organic compounds in high yields and with high isomeric purity. Our new method for making nitronium trifluoromethanesulfonate comprises reacting anhydrous nitric acid with trifluoromethanesulfonic anhydride in an anhydrous solvent such as dichloromethane. The resulting reagent mixture is a homogeneous solution.
2. Description of the Prior Art
A known process for preparing ethynylated biphenyl compounds such as 2,2'-bis(phenylethynyl)-5,5'-diaminobiphenyl appears in U.S. Air Force Report No. AFML-TR-76-198, entitled, "Synthesis of Polimides Curable by Intramolecular Cycloaddition," by Frederick L. Hedberg et al, dated March, 1977. Hedberg et al report an overall yield of 2,2'-bis(phenylethynyl)-5-5'-diaminobiphenyl that is less than 10% by weight of the empirically-predicted yield. And their process is both laborious and costly.
Accordingly, the need exists for a lower cost, higher yield synthesis method for making ethynylated aromatic compounds and particularly amino-substituted ethynylated biphenyl compounds.
A known process for making nitronium trifluoromethanesulfonate appears in an article entitled, "Aromatic Nitration with Nitric Acid and Trifluoromethanesulfonic Acid", by Clifford L. Coon et al, and published in the Journal of Organic Chemistry, Volume 38, No. 25, at pp. 4243-4248 (1973). In their process, trifluoromethanesulfonic acid reacts with anhydrous nitric acid to produce a heterogeneous mixture of nitronium trifluoromethanesulfonate and a hydronium species. These products are extremely difficult to separate from one another. Our new process produces none of the hydronium species, and our product mixture is homogeneous. In practice, our product mixture can be used in precise amounts for certain nitrations that require stringent control of stoichiometry.